The invention relates to a piezoelectric resonator element of crystallographic point group 32, which can be operated as a thickness shear resonator in contact with a carrier medium, and a measuring system for determining at least one chemical, biochemical or physical parameter in a carrier medium, which carrier medium can be brought into contact, on at least one side, with a piezoelectric resonator element of point group 32 having electrical contacts and —if required —at least one sensitive layer, where at least one resonance property, i.e., preferably the resonance frequency of the resonator element operated as a thickness shear resonator provides a measure for the chemical, biochemical or physical parameter to be determined.
Such resonator elements are employed for instance in micro-balances, which are based on the effect that the resonance properties of a piezoelectric resonator, usually a thickness shear resonator, are changed by the mass load on the resonator surface and by the viscosity or the electrical conductivity of the adjacent medium. Microbalances of this sort are frequently used for the in-situ measurement of layer thickness via the mass load. In these applications the micro-balances are operated in vacuum. Recently this technique has also been used for determining the concentration of certain components in liquids or gases, where at least one surface of the resonator is provided with a selectively binding layer, which essentially binds only the substance to be measured to the resonator surface, thus increasing the oscillating mass.
Since the resonance properties of a piezoelectric resonator are not only dependent on the oscillating mass but also on capacitances which act in series with or parallel to the resonator, it is also possible to measure electrical properties such as conductivity or the dielectric constant, respectively changes in these properties, of liquids or gases.
A disadvantage of all these applications lies in the fact, that temperature changes in the resonator itself as well as in the medium to be measured may exert a considerable influence on the resonance properties, especially on the resonance frequency, which either necessitates the use of costly and cumbersome thermostatic devices to counter the effects of temperature fluctuations, or compels one to accept a reduced sensitivity and accuracy of the sensor. From the paper “Oszillatoren für Quartz-Crystal-Microbalance-Sensoren in Flüssigkeiten”, Technisches Messen 65 (1998) by F. Eichelbaum, R. Borngräber, R. Lucklum, P. Hauptmann and S. Rösler, it is for instance known that the temperature dependence of the resonance frequency, i.e., the oscillator frequency of the predominantly used quartz AT-cut resonators, which is very low in air, especially at room temperature, shows a considerable slope of 35 Hz/° C. at a fundamental frequency of 10 MHz in the case of one-sided contact with water. Stated independently of the fundamental frequency this is 3.5 ppm/° C.